If the height of transmitting and receiving antennas are $80\,m$ each,the maximum line of sight distance will be $..............\,km$. Given: Earth's radius $= 6.4 \times 10^6\,m$.

In an amplitude modulation,a modulating signal having amplitude of $X$ Volt is superimposed with a carrier signal of amplitude $Y$ Volt in the first case. In the second case,the same modulating signal is superimposed with a different carrier signal of amplitude $2Y$ Volt. The ratio of the modulation index in the two cases respectively will be:

For efficient transmission of sound waves of wavelength $\lambda$,the minimum length of the antenna should be.....

Match the following:

List-$I$ (Signal)	List-$II$ (Bandwidth)
$i$. Speech Signal	$a$. $4.2 \text{ MHz}$
$ii$. Music Signal	$b$. $6 \text{ MHz}$
$iii$. Video Signal	$c$. $20 \text{ kHz}$
$iv$. $T$.$V$. Signal	$d$. $2.8 \text{ kHz}$

Modulation is a process of superposing

The intensity of a light pulse travelling along a communication channel decreases exponentially with distance $x$ according to the relation $I = I_0 e^{-\alpha x}$,where $I_0$ is the intensity at $x = 0$ and $\alpha$ is the attenuation constant. The attenuation in $dB/km$ for an optical fibre in which the intensity falls by $50$ percent over a distance of $50 \ km$ is